The present invention relates to a position sensor and to a torque sensor including such a position sensor. The torque sensor is suitable for use within a vehicle steering control system such as an electrical power assisted steering (EPAS) system.
SU 517 815 discloses a torque measuring apparatus in which two discs are mounted on a torque transmitting shaft. The discs are spaced apart and each carries two sets of slots. The slots cooperate to define apertures for the passage of light. Light sources and photodetectors are positioned on opposing sides of the discs. The slots are overlapping and arranged such that relative motion between the discs due to a torque in the shaft alters the sizes of the apertures such that apertures associated with one cooperating set of slots increase in size, whereas the other apertures associated with the other cooperating set of slots decrease in size. The intensity of the light transmitted through the apertures is measured by the photodetectors and compared using a bridge circuit.
Such an intensity based arrangement needs shielding from stray light, and also requires matched sources and receivers or careful calibration in order to provide reliable measurements. Furthermore the discs must be manufactured to a high tolerance and be accurately mounted. Otherwise systematic variations in transmitted light intensity will occur due to errors in the width of the slot or run out (i.e. non coaxial alignment) between the discs. EP-A-0 194 930 describes a similar arrangement to that disclosed in SU 517 815.
SU-A-1 364 918 discloses a similar arrangement. However the outputs of the detectors are used to control digital timers. The times recorded on the timers are analyzed to determine the rate of rotation of the shaft and the torque transmitted by the shaft.
According to a first aspect of the invention, thee is provided an optical displacement sensor as defined in the independent claims.
Preferred embodiments of the invention are defined in the claims.
According to a second aspect of the present invention, there is provided a position sensor comprising a multi-element detector responsive to radiation impinging thereon, and a modulator movable with respect to the multi-element detector in response to movement of an input element whose position is to be measured, the modulator being arranged to modulate the radiation pattern incident on the detector.
Preferably a data processor, for example, dedicated hardware or a programmable data processor, is arranged to receive an output of the detector and to determine the position of the modulator by analyzing the output of the detector to locate the position of a transition of the intensity of the radiation incident on the detector between first and second intensities, or the positions of regions of a first intensity and regions of a second intensity.
Advantageously the magnitudes of the first and second intensities are dynamically adjustable. The first intensity may be defined as a range of intensities greater than a first intensity threshold. The second intensity may be a range of intensities less than a second intensity threshold. The second intensity threshold may be a predetermined fraction of the first intensity threshold. Alternatively, the data processor may normalize the output of each element of the detector and the first and second intensities may be judged by comparing the normalized output with a predetermined threshold value.
Preferably the input element is biased towards a first position with respect to a further element. The further element may be an output element of a force transmitting device. The position sensor may be arranged to measure the relative translational or rotational positions of the input and further element. It is thus possible to provide a load or torque sensor.
According to a third aspect of the present invention, there is provided a torque sensor, comprising a first modulating element mounted for rotation and arranged to spatially modulate radiation passing therethrough in accordance with a first modulating pattern, a second modulating element mounted for rotation and arranged to spatially modulate radiation passing therethrough in accordance with a second modulating pattern, the first and second modulating elements being coupled by a torsion member, and being optically in series, and a detector array responsive to a radiation pattern of radiation modulated by the first and second modulating elements.
Preferably the first and second modulating patterns are regular. Preferably the first modulating pattern has a predetermined mark-space ratio. Preferably the second modulating pattern has a predetermined mark-space ratio. Advantageously a period of the first modulating pattern is substantially equal to a period of the second modulating pattern. Advantageously the mark-space ratio of the first modulating pattern is different to that of the second modulating pattern. The periods and mark-space ratios may be measured in terms of distance or in terms of angle measured from an axis of rotation.
Preferably, when the torsion member is substantially rotationally unstressed, the first and second modulating patterns are offset with respect to one another by a predetermined amount. Preferably, when the torsion member is unstressed, the center of a mark in one pattern is substantially coincident with the centre of a space in the other pattern.
Preferably the first and second modulators are mounted on first and second shafts coupled together via the torsion element. The first and second modulators may be discs or cylinders coaxially disposed with respect to the torsion element. Advantageously the first and second modulators have a plurality of slits formed therein defining the marks or spaces of the modulation patterns. Advantageously the slits within each disc are substantially identical. Advantageously the width of first regions defining each end of, and separating neighbouring, slits of the first disc are twice the size of comparable regions of the second disc.
Preferably the spatial extent of the detector array is such that, in use, at least five transitions of the radiation between first and second thresholds will always be detectable by the array.
Preferably the detector array is arranged to be responsive to radiation passing through an area whose extent in a direction (be it linear or curved) parallel to the direction defining the spatial period of the first and second modulating patterns is such that the area is greater than a period of either pattern. Advantageously the detector array, the first modulator and the second modulator are relatively closed spaced and the detector array is longer than the period of the modulating patterns.
Advantageously a signal from each element of the detector array is provided to a data processor. The outputs of the detectors of the array may be output in a serial manner. The data processor may normalize the output of each element of the array to correct for different sensitivities between elements of the array. The data processor may examine the output of the array to determine the radiation pattern incident on the array from positions of transitions of radiation intensity between first and second intensity levels. The data processor may interpolate the intensity data to further refine the estimate of the positions of the transitions.
According to a fourth aspect of the present invention, there is provided a modulator arrangement for an optical position sensor for measuring the relative positions of first and second objects, the arrangement comprising first and second parts coupled together by severable links so as to hold the first and second parts in a predetermined relationship prior to attachment of the first part to the first object and the second part to the second object.
It is thus possible to provide an arrangement in which the relative positions of the first and second parts are accurately maintained during the construction of the optical position sensor. Such an arrangement avoids the intricate and time consuming alignment steps required to produce a reliable position sensor of the type described in SU 517 815.
Preferably the first and second parts define and move over a common surface. Advantageously, when each of the first and second parts in planar, the first and second parts are coplanar. Such an arrangement may reduce the effects of parallax within optical position sensors.
Preferably the severable links are waisted (i.e. narrower) regions of material connecting the first and second parts. The severable links can be broken following the attachment of the first part to the first object and the second part to the second object. The severable links may be broken by the application of stress, by etching, by mechanical cutting, by spark erosion, by laser cutting or by any other suitable process.
Preferably the first and second parts carry respective markers. In an arrangement for measuring limited relative rotary motion, such as in a torque sensor, the first part may comprise a first ring having a plurality of radially extending lands or fingers formed thereon. The lands or fingers may be inwardly extending. The second part may comprise a second ring having a plurality of lands or fingers extending therefrom, with the lands or fingers of the second part being interdigitated with the lands or fingers of the first part. One or both of the first and second rings may be divided into segments. In one embodiment, the second ring is divided into a plurality of segments. Each of the segments is held to the first part by respective severable links, and each segment is arranged to be individually attached to the second object.
According to a fifth aspect of the present invention, there is provided a modulator arrangement for an optical position sensor for measuring the relative positions of first and second object, the arrangement comprising first and second parts which define a common surface, the first part being attached to the first object and the second part being attached to the second object.
Advantageously the first and second parts are co-planar. Advantageously the first and second parts are concentric rings carrying respective markers thereon.